Physical chemical characteristics of the Solnechnoye tin deposit hydrothermal system have been stud ied. Ores of cassiterite-tourmaline type was formed in a vertically dipping fracture zone, extended up to 8 km in a metamorphosed mass of sandstone-shale rocks. This rock mass transfers in its root part to a granitoid massif (quartz diorites-monzonites).The evolution in composition of the mineral-forming solution was studied by gas-chromatographic analysis of gaseous phase of fluid inclusions. The analysis of aqueous extracts from fluid inclusions (AAS, potentiometry, ICP and ionic chromatography) was also performed. It was found that as the process proceeded from the preore quartz-tourmaline stage to productive quartz-cassiterite, the values of axco-' a(s11), C02/CH4, HCO3/Cl and pH increased. The value of foe corresponded to the F-Q-M buffer on thie preore stage and increased up to that of the Ni-NiO buffer during the formation of the main part of cassiterite ores.Study of organic compounds showed that a greater degree of oxidation in comparison to the preore stage is typical for the quartz-cassiterite and the main sulfide stages. It means that organic compounds participated in redox reactions which took place in the solutions during the precipitation of these mineral assemblages.The 8180 values of fluids calculated from the 8180 values obtained for the minerals show a distinct trend of decrease toward later stages. The 8D values for the waters from the inclusions are close to those found for contemporary meteoric waters (-120%o). A transport model of non-isothermal isotope exchange between water and arbitrary number of minerals along the fluid flow paths was used for the interpretation of these isotope data as well as of the data on the isotope composition of the wall-rocks. Descending flow of meteoric waters was found to interact with sandstone-shale rocks and with granitoids before entering into the discharge zone. Thus while studying reasons of ore formation in a given hydrothermal system it is necessary to answer subsequently several questions: a) what are the main changes in the physical chemical prop erties of fluid in the process of the ore formation; b) which of the factors plays the decisive role in the deposition of ore material; c) with which re actions these changes are coupled; and d) which properties of the hydrothermal system are respon sible for the changes of physical chemical condi tions in the solutions. Large economic tin deposit Solnechnoye was chosen for the main object of our study. This de posit is a typical representative of tourmaline type of cassiterite-silicate formation according to the classification (Lugov et al., 1976). Some ques tions of geological structure, chemical and miner alogical compositions of this deposit have been discussed by several authors (Gonevchuk et al., 1984). Many papers (Bannikova et al., 1984(Bannikova et al., , 1989Kokorina, 1982;Sushchevskaya et al., 1978Sushchevskaya et al., , 1984 are dedicated to the problems of physical chemi cal conditions of its for...

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